As the negative electrode for non-aqueous electrolyte secondary batteries, substances capable of absorbing and desorbing lithium ions such as a carbon material have conventionally been used, and lithium ion secondary batteries employing such substances are commercially available. As the binder, styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are typically used. However, non-aqueous electrolyte secondary batteries having sufficient energy density and charge/discharge cycle characteristics have not been achieved yet.
Meanwhile, investigations have been done on the use of materials containing Si as a negative electrode active material that contributes to higher energy density and excellent charge/discharge cycle characteristics. For example, Japanese Laid-Open Patent Publication No. Hei 9-289022 proposes the use of SiO as a negative electrode active material and an acrylic acid polymer as a binder. Likewise, Japanese Laid-Open Patent Publication No. 2005-11802 proposes the use of an alloy containing Si as a negative electrode active material.
However, when the negative electrode of a non-aqueous electrolyte secondary battery contains an active material containing Si having high energy density, the use of conventionally used SBR or CMC cannot yield sufficient binding property, and therefore electrode decay resulting from expansion and contraction during charge/discharge cannot be prevented completely. Likewise, the use of polyacrylic acid having a low weight-average molecular weight cannot yield sufficient binding property, and electrode decay due to expansion and contraction during charge/discharge cannot be prevented completely.
The weight-average molecular weight of polyacrylic acid has a correlation with binding property or viscosity. If the weight-average molecular weight is low, the binding property will be low. If the weight-average molecular weight is high, the viscosity will be high. Accordingly the use of polyacrylic acid having a low weight-average molecular weight cannot yield sufficient binding property, failing to prevent the electrode decay resulting from expansion and contraction during charge/discharge. If the viscosity increases with a high weight-average molecular weight, it makes the dispersion of polyacrylic acid difficult, creating variations in battery characteristics.
When the electrode contains an active material containing Si, the active material reacts with a trace amount of water in the battery during charge/discharge, producing a gas. Because cross-linked polyacrylic acid, lithium salt of cross-linked polyacrylic acid, sodium salt of cross-linked polyacrylic acid and calcium salt of cross-linked polyacrylic acid, lithium salt of non-crosslinked polyacrylic acid, sodium salt of non-crosslinked polyacrylic acid and calcium salt of non-crosslinked polyacrylic acid are highly hygroscopic, they are not suitable for use as binders.
Furthermore, because polymethacrylic acid and polyacrylic ester are soluble in electrolytes, they dissolve in the presence of electrolytes, so that the electrode cannot retain its shape, leading to a significant decrease in charge/discharge cycle characteristics.
When the electrode contains an active material containing Si having high energy density, the electrode expands and contracts during charge/discharge. In the case of an electrode produced by applying, onto a current collector, a slurry (electrode material mixture) prepared by mixing an active material and a binder with a dispersing medium such as water or an organic solvent, a thin active material layer (material mixture layer) formed on the current collector is bonded to the current collector by the binder. Accordingly, even if the active material expands and contracts during charge/discharge, the conductivity is unlikely to decrease.
In the case of an electrode formed of a molded article in the form of a pellet having a certain thickness without a current collector, however, the strength of the molded article lowers due to expansion and contraction of the active material, leading to electrode decay. As a result, the conductivity decreases, and the charge/discharge characteristics decrease significantly.